Real-time coherent optical processor

ABSTRACT

A coherent optical processor comprising writing, developing, readout and erasing stations disposed proximate to and consecutively spaced along the direction of motion of a rotatable drum having a photoconductive layer affixed thereto. A timevarying signal is written on the photoconductive layer by first establishing a uniform electrostatic charge thereon and then applying a light beam to spatially modulate the electrostatic charge pattern in accordance with time-amplitude variations of the light beam. Next it is exposed to a cloud of toner particles at the developing station whereat the spatially distributed electrostatic charge pattern is converted to a visible image of varying transparency. Optical processing is performed as the visible image passes the readout station where it intersects a laser beam and thereafter the toner image is erased from the tape in readiness for the next cycle of operation commencing at the writing station.

United States Patent McMahon et a1.

REAL-TIME COHERENT OPTICAL PROCESSOR Donald Howland McMahon, Carlisle;William Thomas Maloney, Sudbury, both of Mass.

Assignee: Sperry Rand Corporation Filed: Jan. 23, 1970 Appl. No.: 5,437

Inventors:

References Cited UNITED STATES PATENTS 7/1970 Clark et a1. ..355/512/1970 DeBitetto... ..178/7.3 D 9/1966 Wright ..346/74 P PrimaryExaminerBernard Konick Assistant Examiner-Gary M. Hoffman Altomey-S. C.Yeaton [5 7] ABSTRACT A coherent optical processor comprising writing,developing, readout and erasing stations disposed proximate to andconsecutively spaced along the direction of motion of a rotatable drumhaving a photoconductive layer afi'ixed thereto. A timevarying signal iswritten on the photoconductive layer by first establishing a uniformelectrostatic charge thereon and then applying a light beam to spatiallymodulate the electrostatic charge pattern in accordance withtime-amplitude variations of the light beam. Next it is exposed to acloud of toner particles at the developing station whereat the spatiallydistributed electrostatic charge pattern is converted to a visible imageof varying transparency. Optical processing is performed as the visibleimage passes the readout station where it intersects a laser beam andthereafter the toner image is erased from the tape in readiness for thenext cycle of operation commencing at the writing station.

PATENTED JAN25 I972 SI'EUlUFZ LASER II/I FlG.l.

111 v11; 111111111111 1 IIA/I 44 MIRROR FIG.3.

INVENTO/PS DONALD H. MC MAHO/V W/LL/AM T. MALONE).

ATTORNEY PATENTEB M2 5 W2 SBEETZUFZ ATTORNEY REAL-TIME COHERENT OPTICALPROCESSOR BACKGROUND OF THE INVENTION 1. Field of the Invention Thepresent invention relates to optical processors and more particularly toa coherent optical processor utilizing electrophotographic techniquesand having the capability of operating substantially in real-time withmaximum processing delays of about 1 second or less.

2. Description of the Prior Art A coherent optical processor typicallyincludes a transparency on which the signal to be processed is recordedin the form of a two-dimensional pattern of varying opacity or thicknessfor respectively affecting the intensity or phase of a coherent lightbeam propagated therethrough, the recording transparency being used inconjunction with additional components such as lenses andv spatialfilters for performing correlation, pulse compression, spectrum analysisand other functions as is well known to those skilled in the art.

In the early state of the art photographic film was generally used asthe recording means because of its many excellent properties,particularly regarding resolution and sensitivity. Photographic film hasthe disadvantage, however, of not being reusable and further requiresconsiderable development time which precludes its applicability toreal-time (instantaneous) signal-processing operation. In the interestof overcoming these limitations other recording medial have beeninvestigated in recent years. Ultrasonic delay lines and photochromicmaterials in particular have received considerable attention. Both ofthese devices are capable of operating in real-time but each hasinherent limitations. Ultrasonic delay lines, for example, are notsuitable for processing signals having a time duration larger than thetime required for an acoustic wave to propagate from one end of the lineto the other, usually less than I microseconds. Photochromic materials,on the other hand, have the disadvantage compared to other materials ofbeing extremely insensitive to light, particularly in the erasure mode,to the extent that development of a practical device has thus far beenprecluded.

Other materials such as ferroelectrics and arrays of electrooptic andliquid crystals have also been investigated but research in these areasis in the formative stages and therefore it is not known whether thedesired characteristics of reversibility (for repetitive writing anderasing), high sensitivity (for fast writing), minimum delay time (forreal-time processing), large storage capacity (for high resolution),stability and nondestructive readout will be realized with suchmaterials. These characteristics are attainable, however, at the presenttime with a coherent optical processor constructed in accordance withtheelectrophotographic technique of the present invention as will becomeapparent from the subsequent detailed description of the preferredembodiment.

SUMMARY OF THE INVENTION In a preferred embodiment of the presentinvention a signal which is to be processed is temporarily recorded on aphotoconductive medium affixed to a rotatable drum in preparation forpresentation in the path of a light beam emitted from a laser sourceconstituting a part of the processor. Various means are positionedadjacent the drum in spaced relation therealong for performing acomplete operating cycle including precharge, write, develop, processand erase stages which are accomplished in succession as a given regionof the drum moves past discrete stations whereat the respective meansare located. More specifically, at the inception of an operating cycle aprecharging electrode establishes a substantially uniform electrostaticcharge of predetermined polarity on a given region of thephotoconductive medium and thereafter the signal to be processed iswritten thereon by means of an optical signal incident on the drum. Asthe uniformly charged region of the photoconductive medium moves pastthe optical writing station the charge pattern is spatially modulated inaccordance with the time variations of the optical signal. Next, thespatially modulated electrostatic charge pattern is convened to acorresponding visual pattern of varying transparency by exposing thephotoconductive medium at a developing station to toner particles whichare attracted to the electrostatically charged segments of thephotoconductive medium in proportion to the charge existing thereon. Theresulting visual signal pattern is then moved across the propagationalpath of a coherent light beam emitted from a laser source whereupon theoptical processing is performed in conjunction with other conventionalcomponents referred to hereinbefore. As the concluding step of anoperating cycle the toner is brushed away from the tape at an erasurestation preparatory to the beginning of the next cycle.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 3 is a view taken along line 3-3showing the laser source position relative to the embodiment of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, aluminumdrum lilhas a photoconductive layer 11, typically selenium, affixedthereto and is rotated counterclockwise about axis 12 in the directionof arrow 13 by a motor not shown in the drawing. In operation of theprocessor, as the drum rotates past corona charging wire 14, whichextends transverse to the plane of the drawing in conductive housing 16,the adjacent region of the selenium is uniformly electrostaticallycharged to a potential of approximately 700 volts. This is successfullyaccomplished by energizing the charging wire from a +5,000 to +7,500volts DC source. Thereafter, signal information is written on the drumby intensity modulating an optical beam 17 focused on the selenium bymeans of lens 18 which enhances the writing resolution. The opticalwriting beam may be obtained, for example, from the display screen of acathode-ray tube 19 such that successive line scans of the cathode-raybeam in a direction perpendicular to the plane of the drawing producessimilar line scans on the drum, each scan being across a discreteportion of the selenium as a consequence of the rotation of the drum.The optical writing beam has the effect of discharging the selenium byphotoconduction at those points where it is illuminated while leaving itfully charged at the unexposed points. As a result of this action anelectrostatic charge image is recorded on the drum in correspondence tothe applied optical writing signal, that is, the time-varying intensitymodulating optical signal is recorded as a spatially varyingelectrostatic charge pattern.

Upon rotating to the location of developing station 20 the spatiallydistributed charge pattern is converted to an identical visual patternby applying toner (minute particles of carbon impregnated in resin orplastic) to the drum. This can be accomplished by any of varioustechniques well known in the art as, for example, by establishing in thevicinity of the drum periphery a cloud of toner particles having acharge opposite to the charge on the selenium. Preferably, tonerparticles which tend to acquire a negative triboelectric charge are usedand, if desired, additional negative charge may be imparted to the tonerso that it readily adheres to the positively charged segments of thedrum in proportion-to the charge existing thereon while the unchargedsegments, namely, the segments completely discharged by the opticalwriting beam, remain substantially devoid of toner particles. Hence,gray scale recording is possible inasmuch as segments having anintermediate charge level capture some toner but less than segmentswhich are highly charged. An illustrative apparatus for perfonning thedeveloping function comprises a canvas belt 21 which moves in aclockwise direction on discs 22a, 22b, 22c and 22d. Toner is applied tothe belt from a hopper 23. Before entering the developing station anegative charge is established on the toner by carrying it past a wirebrush 24 in gentle contact therewith. In the developing station brushes25 constructed of an insulating material brush the toner away from thecanvas belt so as to form a cloud of negatively charged toner particleswhich are attracted to the charged regions of theselenium asexplainedabove. It should be noted that an image which is a negative ofthat produced by the foregoing procedure could be formed simply byselecting the writing potential and toner particle polarities to achievesuch result.

' After leaving the developing location and rotating slightly further inthe counterclockwise direction the visual pattern recorded on the drumintercepts light beam 26 emitted from laser source 27 whereupondiffracted beam 28 is produced by virtue of reflection from thedeveloped toner image pattern recorded on the drum. Cylindrical lens 29slightly focuses both the incident and reflective light, therebycompensating for the cylindrical shape of the drum surface and furthersegregating undifiracted light from the diffracted light which I isrepresentative of the signal information on the drum. Various opticalprocessing systems can be implemented with this setup. For instance,correlation can be performed by placing a reference pattern such as afilm transparency in the path of the reflected beam, or if desired, inthe path of the incident beam on either side of lens 29. Alternatively,the correlation can be performed by means of a modulated light beam usedin lieu of a reference transparency as is known to those skilled in theart; or, if required, the modulated light source can be used inconjunction with the reference transparency. In any event, it should beunderstood that a laser is not necessarily required as the light source.In many instances it is merely required that the light source bespatially coherent to such an extent that at any instant the phase atany point along one ray of the beam can be determined with reference topoints along the other rays of the beam. In some cases, though, temporalcoherence is required so that the phase at one instant can beascertained with reference to that at another instant.

One example of an application requiring a temporally coherent source isa television system for transmitting threedimensional holographic data.A system of this type can be implemented by scanning a holographicpattern with a needlepoint light beam and applying the signal obtainedtherefrom to the Z axis of cathode-ray tube 19 for the purpose ofintensity modulating the scanning light spot appearing on its screenwhereby the holographic interference pattern is recorded on the seleniumdrum. Upon reaching the readout position where the visual pattern isilluminated by the laser, the reflected diffracted light presents areconstructed wave front which forms a virtual image of the originalholographically recorded object, the temporal coherence of the laserbeing sufficient to present the full depth of the image withoutdistortion.

As the drum rotates further in the direction of arrow 13 the recordedvisual pattern leaves the optical processing or readout location andarrives at an erasure station where brush 31 removes the toner incooperation with a vacuum system (not shown) and thus erases the visualimage in preparation for uniform charging by corona charging wire 14 atthe beginning of the next cycle of operation. To facilitate the removalof this toner image, the erasure station can also include sources ofillumination and/or corona discharge devices positioned betweensuccessive brushes or sets of brushes disposed along the direction ofdrum rotation to regulate the charge remaining on the drum. Althoughmost signalprocessing operations will be performed with the entirerecording pattern instantaneously appearing in the path of laser beam26, it will be appreciated that this is not an absolute limitation onthe time duration of the signal to be processed since the various stagesof the cycle could be performed with the full circumferential length ofthe drum being utilized. A system of the foregoing nature, however, doesrequire a drum precisely machined to optical tolerances for exactconcentric rotation about its center axis. In addition, precisioncylindrical lenses are required to correct for optical field curvatureproduced by the curvature of the drum. These requirements can be erasedto some extent by utilizing a very small section of the drum surface forrecording the signal information. Nevertheless, in many instances itwill be necessary or at least preferable to circumvent these undesirablefeatures of the drum configuration by utilizing the drum in conjunctionwith a continuous loop tape as shown in FIG. 2 wherein identicalcomponents have the same numeral designations.

In this arrangement, after precharging, optical writing and developingas explained with reference to FIG. I, the visual image is contacttransferred to transparent tape 35, which would on aluminum discs 33 and34, simply by moving the tape clockwise, as indicated by arrow 36, incontact with the counterclockwise rotating drum 10. To enhance the imagetransfer the tape is first charged to a potential of approximately+l,000 volts by corona charging wire 37 held at a potential of +5,000volts within conductive housing 39. Transparent plastic tape of highoptical quality has been found suitable for operation in this manner. Itshould be understood that the above-mentioned voltage and other voltagespreviously mentioned herein are intended to be illustrative only,different voltages being suitable depending on the location and spacingof the various components.

After the drum image has been transferred to the tape it is carried to areadout station 38 where it traverses a light beam 40 directed thereonfrom a laser 40' by way of mirror 41 canted 45 to the plane of thedrawing as is more clearly shown in FIG. 3. Optical processing is thenperformed in the manner described with reference to FIG. I, theundiffracted beam 42 being angularly displaced from the diffracted beam43 by an angle 0 as indicated in the drawing. Another use of theprocessor, in addition to the correlation and three-dimensional displayapplications previously mentioned, is indicated by the movable slit 44used in combination with photodetector 45 for the purpose of performingspectrum analysis. Various wavelengths in a given diffraction order areselected simply by moving the slit in the direction of angle 0 along theplane of the drawing.

At the completion of the optical processing the toner image is erasedfrom the tape by brush 46 in readiness for recycling in contact with theselenium drum. In a similar manner, any toner remaining on the drumafter contact with the tape is removed by brush 47.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes within the purviewof the appended claims may be made without departing from the true scopeand spirit of the invention in its broader aspects.

We claim: l. A coherent optical correlator comprising, recording means,means for writing a signal on said recording means in the form of aspatially distributed electrostatic charge pattern varying in accordancewith time-amplitude variations of said writing signal, means fordeveloping the electrostatic charge pattern written on said recordingmeans to convert said charge pattern to a corresponding visual imagerepresentative of said writing signal, means including a light sourcefor providing a spatially coherent light beam directed onto saiddeveloped visual image, light focusing means positioned to receive thespatially coherent light beam subsequent to impinging on said visualimage for separating difi'racted and undiffracted components of thebeam, and means for erasing the signal pattern on said recording meansafter irradiation thereof by said spatially coherent beam. 2. Theapparatus of claim 1 wherein said writing means includes prechargingmeans for initially establishing a substan tially uniform electrostaticcharge in a given region of said recording means in preparation formodulation by said writing signal.

3. The apparatus of claim 2 wherein said recording means comprises aphotoconductive material, said writing means includes means fordirecting a writing light beam onto said photoconductive materialsubsequent to precharging thereof for altering the electrostatic chargeat discrete points in said given region to form the electrostatic chargepattern thereon in accordance with the time-amplitude variations of saidwriting light beam, and said developing means includes means forapplying to said recording means toner particles having a chargeopposite to that of the electrostatic precharge whereby the particlesadhere to discrete points on the recording means in accordance with theelectrostatic charge thereat.

4. A coherent optical correlator comprising a rotatable conductive drumhaving a photoconductive layer formed on the cylindrical peripherythereof,

precharging means positioned adjacent said drum for establishing asubstantially uniform electrostatic charge in a given region of thephotoconductive layer as the drum moves past said precharging means,

writing means spaced from said precharging means in the direction ofmotion of said drum for varying the electrostatic charge at discretepoints in said given region to form an electrostatic charge patterntherein in accordance with time-amplitude variations of a writingsignal,

developing means spaced from said writing means in the direction ofmotion of said recording medium for converting the electrostatic chargepattern formed on said recording medium to a corresponding visual imagerepresentative of said writing signal,

a transparent belt in movable contacting relation with said rotatabledrum,

means for charging a predetermined area of said belt with a chargepolarity opposite to that of the electrostatic charge pattern to effectthe contacting transfer of said visual image from the drum to the belt,

light source means for directing a spatially coherent light beam ontosaid predetermined area of the belt as the transferred visual imagemoves transverse to the propagational path of said coherent light beam,

light-focusing means positioned to receive the spatially coherent lightbeam transmitted through the belt for separating diffracted andundiffracted components of the beam,

means spaced from the intersection of said coherent light beam with saidbelt in the direction of motion thereof for erasing said visual image,and

means for erasing any visual image remaining on the drum after imagetransfer to the belt.

1. A coherent optical correlator comprising, recording means, means forwriting a signal on said recording means in the form of a spatiallydistributed electrostatic charge pattern varying in accordance withtime-amplitude variations of said writing signal, means for developingthe electrostatic charge pattern written on said recording means toconvert said charge pattern to a corresponding visual imagerepresentative of said writing signal, means including a light sourcefor providing a spatially coherent light beam directed onto saiddeveloped visual image, light focusing means positioned to receive thespatially coherent light beam subsequent to impinging on said visualimage for separating diffracted and undiffracted components of the beam,and means for erasing the signal pattern on said recording means afterirradiation thereof by said spatially coherent beam.
 2. The apparatus ofclaim 1 wherein said writing means includes precharging means forinitially establishing a substantially uniform electrostatic charge in agiven region of said recording means in preparation for modulation bysaid writing signal.
 3. The apparatus of claim 2 wherein said recordingmeans comprises a photoconductive material, said writing means includesmeans for directing a writing light beam onto said photoconductivematerial subsequent to precharging thereof for altering theelectrostatic charge at discrete points in said given region to form theelectrostatic charge pattern thereon in accordance with thetime-amplitude variations of said writing light beam, and saiddeveloping means includes means for applying to said recording meanstoner particles having a charge opposite to that of the electrostaticprecharge whereby the particles adhere to discrete points on therecording means in accordance with the electrostatic charge thereat. 4.A coherent optical correlator comprising a rotatable conductive drumhaving a photoconductive layer formed on the cylindrical peripherythereof, precharging means positioned adjacent said drum forestablishing a substantially uniform electrostatic charge in a givenregion of the photoconductive layer as the drum moves past saidprecharging means, writing means spaced from said precharging means inthe direction of motion of said drum for varying the electrostaticcharge at discrete points in said given region to form an electrostaticcharge pattern therein in accordance with time-amplitude variations of awriting signal, developing means spaced from said writing means in thedirection of motion of said recording medium for converting theelectrostatic charge pattern formed on said recording medium to acorresponding visual image representative of said writing signal, atransparent belt in movable contacting relation with said rotatabledrum, means for charging a predetermined area of said belt with a chargepolarity opposite to that of the electrostatic charge pattern to effectthe contacting transfEr of said visual image from the drum to the belt,light source means for directing a spatially coherent light beam ontosaid predetermined area of the belt as the transferred visual imagemoves transverse to the propagational path of said coherent light beam,light-focusing means positioned to receive the spatially coherent lightbeam transmitted through the belt for separating diffracted andundiffracted components of the beam, means spaced from the intersectionof said coherent light beam with said belt in the direction of motionthereof for erasing said visual image, and means for erasing any visualimage remaining on the drum after image transfer to the belt.